The present invention relates to a magnetron for microwave ovens, and in particular, to an improved choke which is fitted in the magnetron to suppress, within a wide band zone, the harmonics generated during operation of the magnetron.
To help understand the present invention, a conventional magnetron and a choke in it shall be described with reference to FIGS. 7 and 8 in the attached drawings.
As shown in FIG. 7, the magnetron for microwave ovens includes: an anode resonant circuit section comprising a cylindrical anode block 5, vanes 6 fixed in a radial arrangement within the anode block 5 and an antenna feeder 7 electrically connecting to an end of a vane 6. A cathode section comprising a coiled filament 8 is set on the axis of the anode block 5 and held by end heads 9 and 10 between them and leads 11; a magnetic circuit comprising an electrode 12 fixed at its top and bottom to the block 5 and includes a permanent magnet 13. On the circumference of the antenna feeder 7 there are assembled a cylindrical metal seal 14, a ceramic member 15, discharge tube 16 and a cap antenna 17; on the circumference of the leads 11 are assembled a cylindrical metal seal 18, a ceramic insulator 19 and a terminal 20. The anode block 5 connects to cooking pins 21.
Upon turning on the microwave oven, the anode resonant circuit section oscillates at a frequency of microwave band, and heats the filament 8 to radiate thermoelectron for dc current to apply between the resonant circuit and the cathode section, the thermoelectron picking up energy from the magnetic force of the magnet 13 to rapidly rotate within the operational space between the vanes 6 and the filament 8.
Further, when the rotational velocity of the thermoelectron is adjusted by means of controlling dc voltage between the permanent magnet and the anode block to synchronize with the phase velocity of the high frequency field electrically oscillating at the resonant circuit of the anode block, the high frequency energy of a predetermined frequency (2450 MHz) is generated to radiate through the antenna 7 to the exterior and apply to the food for cooking.
Also, with the magnetron in operation, the resonant circuit generates in addition to its resonant frequency, a higher harmonic in the frequency of an integral multiple of the resonant frequency, and when directly applied, the harmonic causes noises in other electric instruments. Besides, when the harmonic comes to the frequency of five times the resonant frequency to be equal to the frequency for satellite broadcasts, the harmonic gives rise to a serious interference in radiowave broadcastings. Thus, a magnetron is usually equiped with a choke for suppressing harmonic generation within the device.
Generally one or two such chokes are installed within the cylindrical metal seal 14 as shown in FIG. 7. In the conventional art, a choke has been in use with a construction, as shown in FIG. 8, having a flange 1 and a tube member 2 being integral with the flange 1 on its top side and having the length (1) of one-quarter of the frequency to be suppressed.
The conventional choke construction however allows a choke to suppress only the frequency in one band zone, and thus for suppressing frequencies in a wide zone of bands, the construction requires a number of chokes each having the tube member in a different length to be installed within the seal 14.
However, the inner space within the seal is so limited that the disposition of a number of chokes practically is not feasible.